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We present the first results obtained from the identification of ~30,000sources in the 24mic observations of the COSMOS field at S24>~80muJy. Usingaccurate photo-z's and extrapolations of the counts at faint fluxes we resolvewith unprecedented detail the build-up of the mid-IR background across cosmicages. We find that ~50% and ~80% of the 24mic background intensity originatefrom galaxies at z<~1 and z<~2 respectively, supporting the scenario wherehighly obscured sources at very high redshifts (z>~2) contribute onlymarginally to the CIB. Assuming flux-limited selections at optical wavelengths,we also find that the fraction of i-band sources with 24mic detection stronglyincreases up to z~2 as a consequence of the rapid evolution that star-forminggalaxies have undergone with lookback time. Nonetheless this rising trend showsa clear break at z~1.3, probably due to k-correction effects implied by thecomplexity of SEDs in the mid-IR. Finally, we compare our results with thepredictions from different models of galaxy formation. We note thatsemi-analytical formalisms fail to reproduce the redshift distributionsobserved at 24mic. Furthermore the simulated galaxies exhibit R-K colors muchbluer than observed and the predicted K-band fluxes are systematicallyunderestimated at z>0.5. Unless these discrepancies result from an incorrecttreatment of extinction they may reflect an underestimate of the predicteddensity of high-z massive sources with strong star formation, which would pointto more fundamental processes that are still not fully controlled in thesimulations. The most recent backward evolution scenarios reproduce reasonablywell the flux/redshift distribution of 24mic sources up to z~3, although noneof them is able to exactly match our results at all redshifts. [Abridged]

Deep Spitzer 24mic COSMOS imaging -- I. The evolution of luminous dusty galaxies - Confronting the models